Such heat exchangers may also be referred to as fitted, or mechanically assembled, heat exchangers, because the tube end portions are subsequently expanded, or flared, so as to compress the collar portions of the gasket. This ensures proper sealing of the connection between the tubes and the header plate, without any need for brazing or similar techniques.
It is known to provide heat exchangers of this type in which the end portions of the tubes are deformed prior to being assembled to the header plate, the deformation being such as to give each tube end portion a different cross section from that of the main body of the tube. In one known way of carrying out this deformation, the cross section of each tube end portion is of rounded form, in the manner disclosed in French patent specification No. FR 2 474 674A. In another known method, the cross section of each tube end portion is deformed to an oval shape in the manner described in European patent specification No. 0 387 678. However, this modification of each tube, in its end portion, has the disadvantage that the cold working of the metal of the tube in this way can sometimes lead to fracture of the wall of the tube during the subsequent expansion or flaring operation which secures the tube to the header plate, and so enables the heat exchanger to be assembled mechanically.
In addition, the change of cross section of the tube between the main body of the tube, i.e. the working part of the tube which is surrounded by the fins of the tube bundle, and the tube end portion itself, entails a portion of the tube being unable to have any fins; or, failing that, it means that any fins associated with that tube portion are necessarily deformed and are therefore relatively ineffective.